A typical I/Q modulator or I/Q demodulator implemented with passive components has been implemented using two double-balanced mixers, each including two RF transformers and four diodes. An example of such a generally used demodulator is presented in FIG. 1. Therein an input signal, i.e. intermediate frequency signal IF IN, is divided into two branches and between the branches a 90.degree. phase quadrature is produced in a phase shifting circuit 101. Both of the signals 102 and 103 thus produced, in the present application referred to as mixed signals, are applied to double-balanced mixers 125, 125'. The mixer 125 of one of the branches is composed of balanced transformers 126 and 128, and of a bridge 127 comprising four diodes. After the transformer 126 a mixed signal 112 is connected to the terminals a, b of the diode bridge 127. To the other terminals c, d of the mixer 125, also a signal LO IN of a local oscillator is applied in the form of signal 124 through a power divider 111. The mixing results in an I signal 113 from the mixer 125 which is low pass filtered to provide an output signal I. The operation of the other mixer circuit 125' is equivalent, and respective reference numerals have been marked with an apostrophe('); the mixed signal 103 is connected to the terminals f, b of the diode bridge 125'. A Q signal with a 90.degree. phase quadrature relative to the I signal, is obtained from the other mixer 125', thus providing markings I=In phase and Q=Quadrature (90.degree. phase quadrature).
A demodulator known of prior art may, due to the symmetry in the circuitry, be implemented in a variety of ways in order to lead to the same result. This fact should be borne in mine in studying the description of the present invention, even though it is not separately mentioned in each context. Therefore, for instance a 90.degree. phase quadrature may be produced between the local oscillator signals 124 and 124', whereby no phase difference is produced between the mixed signals 102 and 103 in the circuit 101 serving now as the power divider.
In accordance with the symmetry mentioned above, the operation of a demodulator circuit can analogously be transformed into the operation of a modulator circuit in which also a variety of possibilities of implementing different functions are viable due to the symmetry, as is well known in the art.